The present invention relates to desulfurization systems for a hydrocarbon fuel containing a slight amount of methanol. The present invention also relates to fuel cell systems using a hydrocarbon fuel containing a slight amount of methanol as the raw fuel for the cell systems.
In a fuel cell system, in particular a proton-exchange membrane fuel cell system (PEFC) that has been drastically improved in recent years, it is essential in the process of producing hydrogen from a hydrocarbon raw fuel to remove sulfur compounds contained therein to an extremely low level in order to allow the following catalytic reaction processes such as reforming, water gas shift, and selective CO oxidation to proceed normally for long periods. Therefore, many of the fuel cell systems are provided with a desulfurization section for removing sulfur compounds contained in a raw fuel by adsorbing or hydrocracking. Eligible desulfurizing agents and catalysts include hydrodesulfurizing agents and sulfur adsorbents. Among these, zeolite-based desulfurizing agents supporting silver or copper by ion-exchange, which can remove sulfur compounds down to an extremely low level under mild conditions that are atmospheric pressure and relatively low temperatures close to room temperature are industrially useful and have been used widely in domestic fuel cell systems using a low boiling point hydrocarbon such as natural gas or LP gas (liquefied petroleum gas) as a raw fuel (see, for example, Patent Documents 1 to 5 and Non-Patent Document 1)    (1) Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2001-286753    (2) Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2001-305123    (3) Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2004-168648    (4) Patent Document 4: Japanese Patent Application Laid-Open Publication No. 2004-277747    (5) Patent Document 5: Japanese Patent Application Laid-Open Publication No. 2-73887    (6) Non-Patent Document 1: “Nihonkagakukaishi”, 1981, Vol. 12, pages 1945-1950
However, a slight amount of methanol may often be added in some of the hydrocarbon fuels as the need arises. For example, methanol may be added in an LP gas particularly in order to prevent entrained moisture from freezing during a winter season. It was found that the use of the methanol-added hydrocarbon fuels decreases the performances of the above-described zeolite desulfurizing agent. In the case of coexistence of methanol, the phenomenon of the decreased desulfurization performance is not preferable because the durability of a fuel cell system is impaired. Therefore, a desulfurizing agent has been demanded which has a higher desulfurization performance even though a fuel contains methanol.
Whereas, Patent Document 1 discloses that deposition of silver on a hydrophobic zeolite can obtain a higher desulfurization performance when moisture is mixed in a hydrocarbon fuel. However, the inventors of the present invention have found that the desulfurizing agent of Patent Document 1 exhibits only limited performances if it is used for a hydrocarbon fuel containing methanol.
Therefore, there has been a demand for the development of a desulfurizing agent that is not decreased in performances even if it is used under conditions where a fuel contains methanol and water.